1. Field of the Invention
This invention relates to a coating composition, more particularly to an inorganic coating composition having high-heat-resisting, anti-rusting and anti-corrosive properties.
2. Description of the Related Art
Metallic materials which are used in a high-temperature atmosphere may comprise superalloys, heat-resisting steels and stainless steels. Theses materials generally contain more nickel and chromium so as to achieve high-temperature resistance and high-corrosion resistance. However, such materials are not widely used because of their high costs. It is more common to use low-carbon steels and low-alloy steels, due to their relatively low costs, by applying coatings thereon for protection against corrosion.
Surface coating methods for metals generally comprise gilding, metal spraying, electroplating, ceramic coating, porcelain enamelling and painting. Among these methods, gilding, metal spraying, electroplating, ceramic coating and porcelain enamelling require special instruments and operating conditions, and are not suitable for applying to bulky substances and parts of apparatuses after assembly. The methods of gilding, metal spraying, electroplating, ceramic coating and porcelain enamelling are not convenient as compared to painting. However, organic coating materials which are used conventionally in painting are heat-sensitive and prone to decomposition. They generate smoke when burned or incompletely burned, thereby producing or evolving toxic gases.
Inorganic coating materials generally containing alkali-metal silicate as a binder are advantageous in terms of heat-resistance and non-flammability as compared to the organic coating materials. However, since conventional inorganic coating materials, do not have sufficient heat-resisting properties, they are generally applied to concrete walls and concrete floors of an architecture. Specifically, conventional inorganic coating materials containing alkali-metal silicate have a disadvantage in that the inorganic coating materials require hardening agents, such as fluoride, mineral acid, metal powder, polyvalent metal oxide, and polyvalent metal hydroxide, to eliminate their water soluble characteristics so as to improve the forming properties of their coating films. Another disadvantage thereof is that a high temperature is necessary for hardening and curing such an inorganic material. For example, in Japanese Patent Publication (Kokai) H-2-129269, potassium silicate and ultra fine powdered silicon dioxide are dissolved in water under heat in order to form a binder. The binder is then formulated With inorganic fibrous fine powders and a pigment to form a coating composition. This coating composition needs to be heated at 180.degree. C. for about 30 minutes so as to be cured. Japanese Patent Publication (Kokai) H-3-31367 discloses a heat resistant (650.degree. C.) coating for inorganic materials, such as metals, glass and concrete plates. The coating is of a sprayable water-based alkali-metal silicate and should be heated at 230.degree. C. for about 20 minutes for being cured.
Conventional metal coating techniques need 12 processing steps including pretreatment, such as degreasing, descaling and chemical treatments, in order to achieve an intimate bonding between the coating film and the metal substrate thereof. Particularly, degreasing and descaling are time-consuming and highly influence the durability of the coating film.